Bad Behavior is a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots. It goes far beyond User-Agent and Referer, however. Bad Behavior is available for several PHP-based software packages, and also can be integrated in seconds into any PHP script.
Bad Behavior installs like any other multi-file WordPress plugin. Unzip the bad-behavior.zip file, and you will have a
bad-behavior folder containing all the Bad Behavior files.
Before uploading, edit the
bad-behavior/bad-behavior-wordpress.php file and customize the configuration variables there. When logging is on, all blocked requests will be logged. When verbose logging is on, all requests - successful or not - will be logged. And the logging duration specifies how many days worth of logs will be stored in the database. I recommend not using verbose logging without a really good reason, as your database will fill up fast.